NCBC-667: ensure VBucket.LocatePrimary() does not throw AOOR

Change-Id: I9ebf232afe69109ebe12d1b6de2a232caf8d486a
Reviewed-on: http://review.couchbase.org/41894
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>